High vacuum lamps typically include a light source capsule, such as an arc tube, mounted within an evacuated outer envelope.
Generally, in order to improve the vacuum within the outer envelope, high vacuum lamps, e.g., high intensity discharge lamps, including metal vapor lamps, such as sodium lamps and metal halide lamps, are provided with getter means comprising one or more U-shaped getter rings filled with a fast acting getter, for example, barium-aluminum. The getter rings are normally installed in the lower end of the lamp. To activate the getter, a high frequency induced heating is applied to the metal getter rings by means of a coil slid over the lamp. Activating the getter results in the flashing of the latter.
The metal vapors that evaporate during flashing condense on the glass wall and on other parts of the lamp. During condensing, impurities are absorbed by the getter.
As a result of the condensed metal vapors, the bulb wall is contaminated and the glass surface is made opaque. Furthermore, the flashing causes a darkening near the base of the bulb.
Quite obviously, the prior art lamps described above show the disadvantage that the condensed metal can affect the light intensity of the lamp and also contaminate the lamp parts in the bulb.
A greater disadvantage still, consists in the fact that, due to said darkening, the cosmetic appearance of the lamp is significantly reduced, said darkening causing the idea that, even new, the lamps have already been used before.
It would represent an advance in the art to overcome the above-stated disadvantages.